The present invention relates to hydraulic fluid pressure devices, and more particularly, to hydraulic devices such as motors and pumps in which an inner rotor is eccentrically disposed within an outer rotor, and each of the rotors rotates about its center or axis of rotation, the interengagement of the teeth of the rotors defining expanding and contracting volume chambers.
More specifically, the present invention relates to hydraulic devices of the type described above, including a housing or body defining an inlet port in communication with the expanding volume chambers and an outlet port in communication with the contracting volume chambers.
An example of such a hydraulic device is the "charge pump" or "make-up" pump frequently used as part of a hydrostatic transmission to provide make-up fluid to compensate for leakage losses during operation. This make-up fluid is pumped at a relatively low pressure (typically 150-250 psi, 1.03 .times. 10.sup.6 to 1.72 .times. 10.sup.6 Pa) into a "low pressure" portion of the hydraulic circuit. This charge pump or make-up pump may be driven by a shaft from the main engine and is commonly mounted on the main system pump (a "high pressure" pump) which is typically a reciprocating, axial piston pump. Thus, although it will become apparent that the present invention is well suited for use with either a gerotor pump or a gerotor motor of the type in which each of the rotors rotates about its own axis and utilizes what is frequently referred to as "kidney" porting (generally arcuate inlet and outlet ports), the invention is especially adapted for use with gerotor pumps such as the charge pumps of hydrostatic transmissions, and will be described in connection therewith. Furthermore, although the invention may be utilized with a device which is "unidirectional", it is especially advantageous when used in a hydraulic device which is "bidirectional", i.e., pressurized fluid is pumped from the outlet port in response to rotation of the input shaft in either direction.
In charge pumps of the type described, as well as in most other gerotor devices which use kidney porting, it is highly desirable for the pump output, measured in g.p.m. (cubic meters per minute) to increase in a linear relationship as the input speed (in rpm) increases. However, in prior art gerotor pumps, the output g.p.m. has deviated from the desired linear relation with the input rpm at an undesirably low rpm level. This has been caused primarily by the inability to introduce a sufficient volume of fluid into the expanding volume chambers, through the inlet port defined by the pump body or housing thus causing cavitation and a drop in volumetric efficiency. As a result, in order to achieve a desired pump outlet, it has been necessary to provide a higher input speed for a given pump (which in turn caused higher cavitation and substantial reduction in pump performance and life) or in some cases, even a larger pump of greater capacity. In either case, it has been necessary to use an excessive amount of input energy to achieve the desired output flow rate. A typical prior art inlet port will be illustrated in connection with the subsequent description of the present invention.
It has been possible with prior art inlet ports to achieve fairly good filling characteristics with an inlet port at only one end of the gerotor set in unidirectional devices. It has also been possible with prior art porting arrangements to achieve fairly good filling characteristics on both unidirectional and bidirectional devices by providing inlet ports at both ends of the gerotor, which adds unnecessary complexity and cost to the device.